1. Field of the Invention
The present invention relates to an ultra-precision temperature control system for a machine tool and a control method therefor. More particularly, the present invention relates to a control system for ultra-precisely controlling the temperature of a working fluid used in a machine tool for ultra-precision machining of an optical lens or the like and for, at the same time, measuring and recording temperature data with high accuracy. The present invention also relates to an ultra-precision temperature control method for a machine tool by the above-described control system.
2. Description of the Prior Art
The body of a machine tool for effecting ultra-precision machining of an optical lens or the like must avoid deformation as much as possible. Particularly, it is necessary in order to prevent heat deformation of the machine body to control precisely the temperature of a liquid or gas used as a working fluid for hydrostatic bearings and guide way or a fluid used for a shower system for removing cutting chips. Techniques which have heretofore been employed for such temperature control include a system wherein the range of temperature fluctuations is repeatedly damped by using heat exchangers arranged in a plurality of stages, and another system wherein the range of temperature fluctuations is damped by using a special damping tank (for example, see Japanese Patent Application Post-Exam Publication No. 2-48383 (1990)). However, these control systems aim at minimizing the transient deviation of the supply fluid temperature (i.e., the range of temperature fluctuations).
These control systems are extremely effective when used for systems in which the load on the machine side is constant. However, the conventional control systems cause an increase in the steady-state deviation in regard to controlled systems in which load fluctuations are expected, or which include a large dead time component, that is, a time lag between a change in the input signal and the response to the signal at the output side. The control systems proposed in the past have the unstable nature that the range of fluctuations in the controlled liquid temperature increases due to the time lag, and therefore cannot meet the demand for controlling the liquid temperature precisely. Further, when a machine tool as a controlled system is changed for another, the conventional control systems must change the temperature controller in accordance with the type of machine tool to be used and also need to reset control parameters by obtaining new ones experimentally.
In the meantime, an apparatus for measuring a temperature and storing it in memory is generally provided independently of the temperature controller. High-precision temperature measuring devices having a resolving power of the order of 0.001.degree. C. are now commercially available, for example, a device that converts an electric resistance into a voltage by using a thermocouple or a thermistor and further converts it into a temperature, and a quartz thermometer that utilizes the nature of quartz crystal that the frequency of quartz crystal oscillation regularly changes with temperature.
However, devices such as thermocouple and thermistor are inferior in the linearity of the resistance relative to temperature, that is, the relationship between the resistance and temperature which is mathematically expressed by a linear equation, and these devices are readily affected by a drift or the like. On the other hand, the quartz thermometer is costly because a special sensor and measuring device are required, and it needs data processing to be executed by using a personal computer in order to record the measured data.
Temperature control systems for machine tools are demanded to cope with various kinds of machine tool as controlled system, e.g., systems in which the load is constant, systems in which the load is expected to fluctuate, and systems that have a dead time component, in order to realize ultra-precision machining. Further, it is demanded to construct a flexible control system and minimize the steady-state deviation and transient deviation of temperature.
Recently, temperature control systems for machine tools that perform ultra-precision machining have also been demanded to effect temperature control with a resolving power of 0.001.degree. C., for example, and to process temperature data in units of 0.001.degree. C. However, typical conventional temperature control systems for machine tools lack general-purpose properties and are therefore not usable for other types of machine tool. Further, the conventional systems are incapable of recording and processing data.